The present invention relates to an apparatus for detecting a toner concentration (composition ratio) in developing powder for an electrostatic printing machine which uses the developing powder comprising a mixture of magnetic carrier and toner, especially for detecting a flow condition of the developing powder.
In an apparatus in which an electrostatic latent image is developed by developing powder comprising the mixture of the magnetic carrier and the toner, only the toner is consumed in the developing process but the magnetic carrier is not substantially consumed and it is maintained for reuse. As a result, as the developing powder is used repeatedly, a toner concentration of the developing powder decreases gradually. In order to maintain a high quality of the developed image, however, the toner concentration should be maintained at a fixed value or in a given range.
For this reason, an apparatus for detecting a toner concentration of developing powder has been proposed which comprises a transport means for transporting the developing powder stored in a chamber while magnetically attracting the developing powder, a hollow body, a guide means for diverting a portion of the developing powder from the transport means and directing the diverted developing powder to the hollow body and a response means for responding to a permeability of the developing powder passing through the hollow body, an example of which is U.S. Pat. No. 4,131,081.
In the conventional apparatus, however, the flow of the developing powder in the hollow body is obstructed or stopped when the balance of atmospheric electricity between the developing powders is lost due to high air humidity and change of the flow condition based on the change of the toner concentration.
When the developing powder does not flow, the toner concentration of the developing powder detected by the apparatus is always same and does not respond to that of the developing powder actually developing an electrostatic latent image.
In such case, toner is not supplied to the developing apparatus even though the toner concentration of the developing powder decreases.